Re: prony brake

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Original poster: Steve Conner <steve@xxxxxxxxxxxx>

If you connect enough capacitive load to an induction motor, it will 
self-excite (building up from residual magnetism in the rotor) and 
act as a generator. The output voltage is quite poorly regulated. It 
slips just like an induction motor only backwards: the output 
frequency is slightly lower than the shaft RPM would imply. I don't 
know how something spinning at one frequency can generate a current 
at a different frequency, but it just does.

I believe many of the real cheap and nasty petrol powered generators 
use that design rather than a "proper" synchronous alternator, and I 
know for a fact they use it in low-tech small-scale hydro power 
stations. You can buy an induction generator controller for these, 
that burns off the surplus power in a dump load to keep the frequency 
and voltage within limits. The link below shows a commercial unit but 
you can also obtain (free?) plans and build one yourself if you're interested."

   Your reference to slip in the first paragraph is the clue to 
operation.  The slip in the case of a generator is "negative", in 
that the motor shaft is running at a higher speed (the sum of the 
excitation and slip speeds) than the line, rather than lower in the 
case of the motor.  As a consequence, the motor acts as a load on the 
prime mover rather than delivering torque to it.  When it's running 
slow it delivers power to the shaft, when it's running at synchronous 
speed there is no power flow, and when it's running fast it absorbs 
power from the shaft and delivers it to the line. The induction 
generator is handy for small installations which feed a power line or 
feed power into a grid as it's easy to control the amount of power delievered.

Ed